Sodium ion batteries (NIBs, NABs, SIBs) are primarily an alternative to lithium ion batteries (LIBs) for stationary and municipal applications, where system cost may trump energy density per se. However, with the emergence of new electrode materials and new electrolyte additives, there is the possibility for NIBs to match LIBs in performance as well. Hybrid ion capacitors (HICs) are attracting increasing scientific attention since they promise to span the divide between batteries and supercapacitors. A sodium ion capacitor (NIC) is a hybrid device employing Na-ions as the positive charge carriers. Overall the NIC field is quite young, with the first known device being published only several years ago.
Sodium ion anode candidate materials include a range of carbons, titanium based compounds, metals/alloy, oxides and sulfides. Carbon is perhaps one of the more attractive candidates based on its cost, environmental benignness, and the fact that a carbon-based anode is already the standard for commercial LIBs. While Na does not intercalate reversibly into graphite, a range of non-graphitic carbons have been developed instead. Pyrolyzed glucose derived hollow carbon spheres, biomass derived carbon nanosheets, polyaniline derived hollow carbon tubes, and functionalized graphene, functionalized high-surface-area nanocellular carbon foams etc. have been prepared and tested for Na ion storage with different voltage windows.
Nitrogen heteroatoms are well known to provide additional charge storage capacity through reversible binding of the Li (i.e., for LIBs) to the N-based moieties and/or associated defects in the carbons. Oxygen functional groups on the carbon surface are known to provide extra reversible capacity, especially in the high voltage region (1.5-4.5 V vs. Li/Li+). Recently, researchers have demonstrated that the heteroatom (N, O, S, F) functionalization strategy will also work for Na-ion storage. Materials such as nitrogen functionalized carbon nanofiber webs and oxygen functionalized carbon nanosheets were prepared and tested in various Na-ion based energy storage devices.
Nanoporosity is important for high-rate performance of NIB anodes in general. Open porosity is necessary to minimize the solid-state diffusion distances of the Na, by reducing the effective cross-section of the material. Pores also add short circuit paths for Na surface diffusion. With pyrolysis-derived carbons, it is a major challenge to achieve a high surface area while preserving substantial heteroatom content. The high temperatures employed for localized carbon gasification to create the nanopores lead to concomitant elimination of heteroatoms. Instead, hard templating strategies are used to introduce surface areas in the range of 400-800 m2g−1, with the maximum preservation of the heteroatoms. These are relatively complex synthesis methods, which are challenging to implement beyond the laboratory scale. Conversely, standard chemical activation such as by KOH, will significantly eliminate the surface heteroatoms (especially nitrogen) during the process. A scalable and facile methodology to synthesize carbons with high heteroatom content and high levels of nanoporosity remains an essential challenge.